This invention relates generally to lighting units, and more particularly to luminaires adapted to be mounted in a paved horizontal surface such as an airport taxiway.
Currently airport taxiway lights or markers are installed along the center lines of taxiways and at parking areas or aprons to guide pilots from runaway turn-offs to positions within parking areas near terminal buildings, and also to provide guidance to ground crew personnel. The lights are embedded as close as possible to flushness with the pavement of the taxiway so as to permit grading of snow with minimal damage to the light and to permit aircraft and ground crew vehicles to pass directly over the light. The guidance function of the light is carried out by customary taxiway lights by a main prismatic lens which concentrates light in a limited vertically narrow equatorial zone extending circumferentially around the light source at a slight angle to the horizontal plane of the runway surface. It is important that the lower extent of the zone be as shallow as possible, such as 5.degree. above horizontal, to provide guidance to ground crew personnel, such as air marshalls, who direct aircraft into specific positions within parking areas. Usually, the zone extends through a band width of about 10.degree. to approximately 15.degree. above the horizontal.
Known conventional taxiway lighting units employ a housing having upper and lower sections removably joined together by a fluid-tight seal, the upper housing section having a central vertical opening, and an optical system secured within the opening. The optical system includes a main lens, mounted within the opening, having a light exit face of a truncated conical or cylindrical form and a light entrance face of a stepped or prismatic form, and a lamp disposed relative to the main lens to provide illumination of the aforementioned shallow equatorial zone primarily by refraction of light rays emmitted directly from the lamp against the light entrance surface of the lens. Reflectors of a spherical or parabaloid shape are disposed generally about the filament of the lamp to direct additional light rays diverging from the filament upwardly against the lens.
The upper housing section of such conventional taxiway or runway markers is provided with a plurality of relatively high radially-disposed upstanding fin or rib members which serve to protect the lens, that is, which permit aircraft and vehicle wheels and snow-plow blades to pass over the unit without damage to the lens. The upstanding ribs of known markers extend radially outwardly from positions proximate the outer circumference of the lens. The top of the lens is sometimes protected by a metal top cover secured to the fins by a plurality of bolts. Gaskets are disposed between the upper end of the lens and the top cover plate and between the lower end of the lens and the housing upper section. The upper surface of the top plate of known units extends usually 3/4 to 1 inch above the horizontal plane through the surrounding pavement surface, and thus provides substantial obstruction to vehicular wheels and to snow-plow blades.
In another type of conventional taxiway light, the radially disposed ribs extend over the top of the lens and join at a central position above the lens, and are strengthened by additional rib members connecting between the ribs at circumferential positions lying generally overhead the outer circumference of the lens. The rib structure of this latter type of light is integrally molded with the upper housing section and has an overall height of approximately 1 inch above the surrounding pavement surface.
The first-mentioned type of marker which employs a top plate above the lens is adapted to produce illumination only in the shallow equatorial zone. It does not provide illumination directly overhead the lens due to the blockage of light by the top plate. In contrast, the latter-mentioned type produces overhead illumination, in addition to the low angle equatorial illumination. The overhead light is beneficial in apron or parking areas because it illuminates the undersides of aircraft above the marker and thus gives ground crew personnel, such as air marshalls, a reference or fixation with which such personnel can guide the aircraft into an exact position in the parking area. However, the radial and circumferential network of ribbing above the latter marker blocks a substantial portion of the light which would otherwise be projected into the overhead zone.
Furthermore, the relatively high, radially disposed upstanding ribs or fins of all known conventional taxiway marker lights block or shadow a substantial portion of the low angle illumination provided by the optical system therein. Specifically, the fins are of substantial thickness and height (usually in excess of 3/4 to 1 inch) at positions adjacent the lens, with the fins sloping radially downwardly to form a straight inclined upper edge. Light proceeding from the lens in a radial direction is shadowed by the thickness of the fins and in an off-radial or skewed direction by the sides of the upstanding fins. The light distribution pattern of the conventional marker is thus not homogeneous or uniform about the optical system of the unit, but rather varies circumferentially according to the shadowing caused by the protective upstanding fins.
The rib structures and/or cover plate assembly of the conventional markers provide many sharp and deep corners or crevices in which dirt and other contaminates may accumulate and from which such dirt is difficult to remove or clean. Specifically, dirt accumulates at the juncture of the fins and the light exit face of the lens and thereby reduces or diffuses the illumination provided by the lens.
The present invention provides a taxiway marker or light which overcomes the aforementioned problems.